U.S. patent number 4,399,027 [Application Number 06/182,524] was granted by the patent office on 1983-08-16 for flotation apparatus and method for achieving flotation in a centrifugal field.
This patent grant is currently assigned to University of Utah Research Foundation. Invention is credited to Jan D. Miller.
United States Patent |
4,399,027 |
Miller |
August 16, 1983 |
**Please see images for:
( Certificate of Correction ) ** |
Flotation apparatus and method for achieving flotation in a
centrifugal field
Abstract
A gas-sparged hydrocyclone apparatus and method for achieving
separation by flotation in a centrifugal field. The hydrocyclone
apparatus is suitably modified so that a gas phase may be dispersed
into the liquid vortex created in the hydrocyclone.
Inventors: |
Miller; Jan D. (Salt Lake City,
UT) |
Assignee: |
University of Utah Research
Foundation (Salt Lake City, UT)
|
Family
ID: |
22668839 |
Appl.
No.: |
06/182,524 |
Filed: |
August 29, 1980 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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94521 |
Nov 15, 1979 |
4279743 |
Jul 21, 1981 |
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Current U.S.
Class: |
209/164; 209/724;
209/168; 209/170; 209/157; 209/730 |
Current CPC
Class: |
B04C
7/00 (20130101); B03D 1/1493 (20130101); B04C
9/00 (20130101); B04C 5/10 (20130101); B03D
1/1425 (20130101); B03D 1/1431 (20130101) |
Current International
Class: |
B03D
1/14 (20060101); B04C 5/00 (20060101); B04C
9/00 (20060101); B04C 5/10 (20060101); B04C
7/00 (20060101); B03D 001/02 (); B03D 001/14 () |
Field of
Search: |
;209/12,18,144,164,170,211,168,173 ;210/512.1,788 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2748478 |
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May 1978 |
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DE |
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1022375 |
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Mar 1953 |
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FR |
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2263036 |
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Mar 1975 |
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FR |
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1005479 |
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Sep 1965 |
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GB |
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1177176 |
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Jan 1970 |
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GB |
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1500117 |
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Feb 1978 |
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GB |
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545385 |
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Mar 1977 |
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SU |
|
751437 |
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Jul 1980 |
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SU |
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Primary Examiner: Hill; Ralph J.
Attorney, Agent or Firm: Workman; H. Ross Jensen; Allen R.
Hulse; Dale E.
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part application of my
copending application Ser. No. 094,521 filed Nov. 15, 1979 for
AIR-SPARGED HYDROCYCLONE AND METHOD, which issued as U.S. Pat. No.
4,279,743 on July 21, 1981.
Claims
What is claimed and desired to be secured by United States Letters
Patent is:
1. A flotation apparatus for obtaining separation of fine particles
in a centrifugal field comprising:
a chamber having a generally circular cross-section and receiving a
particulate suspension therein, a substantial portion of the
particles in the particulate suspension being fine particles and at
least a portion of the particles in the particulate suspension
being hydrophobic;
inlet means for introducing a fluid under pressure into the
chamber, said fluid being introduced in a generally tangential
fashion thereby creating a vortex in the chamber, the vortex
forming a centrifugal field;
a porous wall forming at least a portion of the outer wall of the
chamber, said porous wall being capable of introducing gas in
finely dispersed bubbles into the vortex of said chamber, said gas
forming bubble/particle aggregates with the hydrophobic particles
in the particulate suspension and said bubble/particle aggregates
being floated towards the core of the chamber and there collecting
to form a froth phase within the core of the chamber, thereby
achieving separation of said hydrophobic particles by flotation in
the centrifugal field;
a gas plenum enclosing the porous wall portion of the outer wall of
the chamber, said gas plenum supplying the gas introduced through
the porous wall into the chamber; and
a vortex finder for directing the froth phase out of the chamber,
said vortex finder being positioned at an upper end of the chamber
and oriented coaxially with the chamber, said vortex finder having
a substantially reduced cross-sectional area as compared to the
chamber.
2. The flotation apparatus defined in claim 1 wherein the chamber
comprises a vertically oriented, cylindricoconical vessel having a
cylindrical section adjacent the inlet means and tapering
downwardly into a frustoconical section.
3. The flotation apparatus defined in claim 1 wherein the fluid is
introduced into the chamber at a flow rate sufficient to create a
centrifugal field in the range of about 80 G in the chamber.
4. The flotation apparatus defined in claim 1 wherein the chamber
comprises a cylindrical vessel.
5. A method for separating fine hydrophobic particles by flotation
in a centrifugal field comprising:
introducing a fluid suspension of particles coaxially into a
cylindrical vessel at a first end, a substantial portion of the
particles in the fluid suspension being fine particles and at least
a portion of the particles in the fluid suspension being
hydrophobic;
creating a vortex in the vessel by introducing a second fluid into
the vessel adjacent a second end in a generally tangential fashion
and removing fluid from the vessel adjacent the first end in a
generally tangential fashion, the vortex forming a centrifugal
field in the vessel;
sparging gas through a porous wall formed in at least a portion of
the outer wall of the vessel, the gas forming finely dispersed
bubbles which form bubble/particle aggregates with the hydrophobic
particles in the fluid suspension;
floating said bubble/particle aggregates towards the core of the
vessel; and
collecting the bubble/particle aggregates at the core of the vessel
to create a froth phase, thereby achieving separation of said
hydrophobic particles by flotation in the centrifugal field.
6. The method as defined in claim 5 further comprising the step of
removing the froth phase coaxially from the vessel at the second
end.
7. A flotation apparatus for obtaining separation of fine particles
in a centrifugal field comprising:
a chamber having a generally circular cross-section;
a coaxial inlet at a first end of the chamber for introducing a
particulate suspension into the chamber, a substantial portion of
the particles in the particulate suspension being fine particles
and at least a portion of the particles in the particulate
suspension being hydrophobic;
a coaxial outlet at a second end of the chamber for removing a
froth phase from the chamber, said coaxial outlet having a
substantially reduced cross-sectional area as compared to the
chamber;
means for introducing a fluid under pressure into the chamber, said
fluid being introduced in a generally tangential fashion thereby
creating a vortex in the chamber, the vortex forming a centrifugal
field;
a porous wall forming at least a portion of the outer wall of the
chamber, said porous wall being capable of introducing gas in
finely dispersed bubbles into the vortex of said chamber, said gas
forming bubble/particle aggregates with the hydrophobic particles
in the particulate suspension and said bubble/particle aggregates
being floated towards the core of the chamber and there collecting
to form the froth phase within the core of the chamber, thereby
achieving separation of said hydrophobic particles by flotation in
the centrifugal field; and
a gas plenum enclosing the porous wall portion of the outer wall of
the chamber, said gas plenum supplying the gas introduced through
the porous wall into the chamber.
8. The flotation apparatus defined in claim 7 wherein the chamber
comprises a generally cylindrical vessel.
9. The flotation apparatus defined in claim 8 wherein said fluid
introducing means comprises a generally tangential inlet adjacent
said second end and a generally tangential discharge adjacent said
first end.
10. The flotation apparatus defined in claim 9 wherein said fluid
introducing means comprises a generally tangential inlet adjacent
said first end and a generally tangential discharge adjacent said
second end.
11. An air-sparged hydrocyclone comprising:
a generally cylindrical vessel;
a coaxial feed at a first end of the vessel for receiving a
particulate suspension into the vessel, a substantial portion of
the particles in the particulate suspension being fine particles
and at least a portion of the particles in the particulate
suspension being hydrophobic;
a coaxial discharge at a second end of the vessel for allowing
removal of a froth phase from the vessel, said coaxial discharge
having a substantially reduced cross-sectional area as compared to
the vessel;
an inlet means for introducing a washing medium into the vessel,
the inlet means being located adjacent and generally tangential to
the second end of the vessel;
an outlet means for removing particles and washing medium from the
vessel, the outlet means being located adjacent and generally
tangential to the first end of the vessel;
a porous wall forming at least a portion of the outer wall of the
vessel between the inlet means and the outlet means, said porous
wall being capable of introducing air in finely dispersed bubbles
into the vessel, the air forming bubble/particle aggregates with
the hydrophobic particles in the particulate suspension and said
bubble/particle aggregates being floated towards the core of the
vessel and there collecting to form the froth phase within the core
of the vessel, thereby achieving separation of the hydrophobic
particles by flotation in a centrifugal field; and
an air plenum enclosing the porous wall portion of the outer wall
of the vessel.
12. A method for separating fine particles in a fluid suspension of
particles comprising:
obtaining a vessel having a circular cross-section;
forming a porous wall in at least a portion of the outer wall of
the vessel;
enclosing the porous wall in a gas plenum;
introducing a feed into the vessel, the feed including particles in
a fluid suspension, a substantial portion of said particles being
fine particles and at least a portion of said particles being
hydrophobic;
providing an outlet means for removing a material from the
vessel;
creating a centrifugal field in the vessel by creating a vortex in
the vessel;
sparging gas from the gas plenum through the porous wall and into
the vortex, said gas forming finely dispersed bubbles which form
bubble/particle aggregates with the hydrophobic particles in the
fluid suspension; and
floating said bubble/particle aggregates towards the core of the
vessel and there collecting the bubble/particle aggregates to form
a froth phase within the core of the vessel, thereby achieving
separation of said hydrophobic particles by flotation in the
centrifugal field.
13. The method defined in claim 12 wherein the obtaining step
comprises preparing said vessel with a cylindrical section and a
conical section and orienting said vessel in a vertical orientation
with said conical section providing a downward taper to the
vessel.
14. The method defined in claim 13 wherein said creating step
further comprises creating said vortex and said centrifugal field
in said vessel by injecting said feed into said cylindrical section
of said vessel in a generally tangential fashion.
15. The method defined in claim 12 wherein the obtaining step
comprises preparing said vessel as a cylindrical chamber.
16. The method defined in claim 15 wherein the introducing step
further comprises introducing said feed into said vessel through a
coaxial inlet and said creating step further comprises creating
said centrifugal field by injecting a second fluid into the vessel
in a generally tangential fashion.
17. The method defined in claim 12 further comprising the step of
removing the froth phase coaxially from the vessel through a vortex
finder positioned at an upper end of the vessel, said vortex finder
having a substantially reduced cross-sectional area as compared to
the vessel.
18. The method defined in claim 12 wherein the feed is introduced
into the vessel at a flow rate sufficient to create a centrifugal
field in the range of about 80 G in the vessel.
19. A gas-sparged hydrocyclone for obtaining separation of fine
particles in a centrifugal field comprising:
a vertically oriented chamber, the chamber having a circular
cross-section;
inlet means for introducing a particulate suspension comprising
hydrophobic particles into the chamber, a substantial portion of
the particles in the particulate suspension being fine particles,
the inlet means comprising a generally tangential entry and the
tangential entry imparting a vortex flow to the particulate
suspension, thereby creating a centrifugal field in the
chamber;
an overflow means for directing a froth phase out of the chamber,
the overflow means comprising a vortex finder located at an upper
end of the chamber and oriented coaxially with the chamber, said
vortex finder having a substantially reduced cross-sectional area
as compared to the chamber;
an outlet means for removing an underflow product from the chamber,
the outlet means comprising a discharge outlet at a lower end of
the chamber and oriented coaxially with the chamber; and
gas sparging means for introducing a gas into the chamber
comprising a gas plenum surrounding the chamber and a porous wall
between the gas plenum and the chamber for introducing gas from the
gas plenum into the chamber, the gas forming finely dispersed
bubbles which form bubble/particle aggregates with the hydrophobic
particles in the particulate suspension, said bubble/particle
aggregates being floated towards the core of the chamber and there
collecting to form the froth phase within the core of the chamber,
thereby achieving separation of said hydrophobic particles by
flotation in the centrifugal field.
20. The gas-sparged hydrocyclone defined in claim 19 wherein the
chamber comprises an upper cylindrical section and a lower
downwardly tapered conical section.
Description
BACKGROUND
1. Field of the Invention
This invention relates to a novel flotation apparatus and method
and, more particularly, to a novel flotation apparatus and method
for achieving flotation in a centrifugal field.
2. The Prior Art
Flotation--General Discussion
Flotation is a process in which the apparent density of one
particulate constituent of a suspension of divided particles is
reduced by the adhesion of gas bubbles to that respective
particulate constituent. The buoyancy of the bubble/particle
aggregate is such that it rises to the surface and is thereby
separated by gravity from the remaining particulate constituents,
which do not attract air, and which, therefore, remain suspended in
the liquid phase. The preferred method for removing the floated
material is to form a froth, or foam, to collect the
bubble/particle aggregates. The froth with collected
bubble/particle aggregates is removed from the top of the
suspension. This process is called froth flotation and is conducted
as a continuous process in equipment called flotation cells.
Importantly, froth flotation is favored by copious quantities of
small, one to two millimeter bubbles.
Conventionally, the success of flotation depends on controlling
conditions in the suspension so that air is selectively retained by
one constituent and rejected by the others. To attain this
objective, the pulp must be treated by the addition of small
amounts of known chemicals which render one constituent floatable
with respect to the remaining constituents. Thus, a complete
flotation process is conducted in several steps: (1) the feed is
ground, usually to a size less than about 28 mesh; (2) a slurry
containing about 5 to 40 percent solids in water is prepared; (3)
the necessary chemicals are added and sufficient agitation and time
provided to distribute the chemicals on the surface of the
particles to be floated; (4) the treated slurry is aerated in a
flotation cell by agitation in the presence of a stream of air or
by blowing air in fine streams through the pulp; and (5) the
aerated particles in the froth are withdrawn from the top of the
cell as a froth product (frequently as the concentrate) and the
remaining solids and water are discharged from the bottom of the
cell (frequently as the tailing product).
Chemicals useful in creating the froth phase for the flotation
process are commonly referred to as frothers. The most common
frothers are short chain alcohols such as methyl isobutyl carbinol,
pine oil, cresylic acid, and the like. The criteria for a good
frother revolves around the criteria of solubility, toughness,
texture, froth breakage, and non-collecting techniques. In
practical flotation tests, the size, number, and stability of the
bubbles during flotation may be optimized at given frother
concentrations.
Much scientific endeavor has been expended toward analyzing the
various factors which relate to improving the conditions during
flotation for improved recovery of particles. One particular
phenomenon that has been known for some time is the poor flotation
response of fine particles. For example, the state of the art is
adequately described in FIG. 1 wherein a comparison is made of the
percentage of recovery from specified size fractions versus the
average particle size for the conventional flotation of certain
sulfide minerals. It will be noted that below about ten microns,
there is an abrupt drop in the percentage of recovery of these fine
particles. In particular, FIG. 1 illustrates size-by-size recovery
curves for a variety of sulfide minerals. Each curve is the result
of a one minute float of a full flotation size range in a timed
batch test (60 seconds), each test being carried out so far as is
possible under the same flotation conditions (i.e., conditioning
and flotation which would lead to good recovery of intermediate
size particles after several minutes flotation time). The
difference in coarse particle recovery between galena and pyrite
might be explained by the density differences between the minerals
(7500 and 5000 kg/m, respectively); however, the same explanation
cannot be offered in the case of pentlandite which has nearly the
same density as pyrite. It is important to note from FIG. 1 that
there is a marked decrease in recovery percentage for these sulfide
minerals at particle sizes less than about 15 microns and further
that this effect is recognized to be generally true for all
particle types.
Basically, surface chemical factors determine the potentiality for
formation of a bubble/particle aggregate. The qualitative
interrelationships between hydrophobicity, contact angle, and
flotation response are fairly well understood but there is little
quantitative information available on the relationship between
hydrophobicity and induction time. Induction time can be defined as
the time taken for a bubble to form a three-phase contact at a
solid surface after initial bubble/particle collision.
Alternatively, it can be regarded as the time taken after collision
for the liquid film between a particle and bubble to thin to its
rupture thickness. Induction times which are characteristics of
good flotation conditions are known to be of the order of 10
milliseconds. However, whereas contact angle appears to be an
intrinsic characteristic of the surface chemical forces, in an
actual flotation system, induction time besides being dependent on
surface chemical forces, is additionally contingent on physical
factors such as particle size, temperature in some circumstances,
and also, because of its nature, presumably on inertial effects.
Consequently, in considering bubble/particle contact and adhesion,
any calculations involving an induction time factor must to some
extent be speculative, but nevertheless may provide a useful guide
to the significance of that factor on affecting flotation rates and
the general flotation response of any particle.
Additional discussions relating to flotation and fine particles
processing may be found in the publications:
Flotation, vols. 1 and 2 M. C. Furstenau, editor, American
Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.,
New York, N.Y., 1976; and
Fine Particles Processing, Proceedings of the International
Symposium on Fine Particles Processing, Las Vegas, Nev., Feb.
24-28, 1980 (vols. 1 and 2; P. Somasundaran, editor, American
Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.,
New York, N.Y., 1980.
In addition to conventional froth flotation, variations in
flotation techniques include the addition of an emulsion of oil.
For example, the separation of coal is greatly assisted by the
addition of about three to five percent or more oil to enhance the
formation of oil droplet/coal particle aggregates. A slurry of
ground coal is flocculated with the oil and the flocs which float
are separated from the refuse material by skimming from the
surface. While this technique does not utilize air bubbles for
flotation, the adaptation of this system to froth flotation has
been used both for coal and a variety of ores such as manganese
dioxide and ilmenite (an oxide mineral of iron and titanium). In
this latter process, a collector and fuel oil are added to the ore
slurry, often with an emulsifier. The conditions of the process are
adjusted so that when the pulp is aerated, the dispersed
oil/particle suspension inverts from that of oil-in-water in the
pulp to one of water-in-oil in the froth. This process, therefore,
occupies a middle position between froth flotation and the
foregoing oil flotation process. Advantageously, the quantity of
oil used is usually much lower than that used for the bulk oil or
spherical agglomeration process, generally only one to several
pounds of oil per ton of ore processed. The modifications of
conventional froth flotation are referred to in the art as emulsion
or oil flotation.
Since for effective aeration the particles should be small and the
original density of the floated material is not too critical,
flotation can be applied where conventional gravity separation
techniques fail. Indeed, so successful and versatile has flotation
become that it has supplanted the older gravity separation methods
in a number of separation problems. Originally, flotation was used
to separate sulphide ores of copper, lead and zinc from associated
gangue mineral particles but is also used for concentrating
nonsulphide ores, for cleaning coal, for separating salts from
their mother liquors, and for recovering elements such as sulphur
and graphite.
Cyclonic Separators--General Discussion
The cyclonic separator or hydrocyclone is a piece of equipment
which utilizes fluid pressure energy to create rotational fluid
motion. This rotational motion causes relative movement of
particles suspended in the fluid thus permitting separation of
particles, one from another or from the fluid. The rotational fluid
motion is produced by tangential injection of fluid under pressure
into a vessel. The vessel at the point of entry for the fluid is
usually cylindrical and can remain cylindrical over its entire
length though it is more usual for it to become conical. In many
instances, the hydrocyclone is used successfully for dewatering a
suspension or for making a size separation (classifying
hydrocyclone). However, equally important is its use as a gravity
separator. Hydrocyclones have been used extensively as gravity
separators in coal preparation plants and design features have been
established for such applications which emphasize the difference in
particle gravity rather than the differences in particle size. Two
general categories of hydrocyclones used for gravity separation can
be distinguished by their design features particularly with respect
to their feed and discharge ports and, to a lesser extent, by the
presence or absence of a conical section.
The first type of hydrocyclone generally has three inlet and outlet
ports and consists of a cylindrical vessel ranging, as found in
industry, from 2 to 24 inches in diameter with a conical or
bowl-shaped bottom. Variations exist in the shape, dimensions,
bottom design, vortex finder, etc. Choice of the various parameters
of the cyclone depend upon the size of the particles to be treated
and the efficiency desired. Thus, the major operating variables of
the hydrocyclone are: the vertical clearance between the lower
orifice edge of the vortex finder and the cyclone bottom; vortex
finder diameter; apex diameter; concentration of feed solids; and
inlet pressure.
In operation, the particle/water slurry is introduced tangentially
and under pressure into the cylindrical section of the cyclone
where centrifugal force acts on the particles in proportion to
their mass. As the slurry moves downward into the conical section
of the cyclone, the centrifugal force acting on the particles
increases with decreasing radii. In such a regime, the heavy
density particles of a given size move outward toward the
descending water spiral much more rapidly than their lighter
density counterparts. Consequently, as these lighter density
particles approach the apex of the cone, they are drawn into an
upwardly flowing, inner water spiral which envelopes a central air
core and these lighter density particles report to the vortex
finder as overflow product. The heavier particles in the outer
spiral along the cyclone wall report to the apex orifice of the
hydrocyclone as an underflow product. Admittedly, this is an
oversimplified description of the separation affected in a
hydrocyclone which is, in fact, a very complex interaction of many
physical phenomena including centrifugal acceleration, centripetal
drag of the fluid, and mutual impact of particles.
The second type of hydrocyclone used for gravity separation has
four inlet/outlet ports and consists of a straight-wall cylindrical
vessel of specified length and diameter and is usually operated at
various inclined positions ranging between the horizontal and the
vertical. A suspension of particles enters the vessel through a
coaxial feed pipe, generally at the upper end of the vessel, while
a second fluid, water or a heavy media suspension, enters the
vessel tangentially, under pressure, through an inlet adjacent the
lower end of the vessel. The pumped medium thus introduced creates
a completely open vortex within the vessel as it transverses the
vessel toward a tangential sink discharge adjacent the upper or
inlet end. The cyclonic action created in the vessel transports the
heavier particles to the sink discharge while the lower density
particles are removed from the vessel through a coaxial outlet
(vortex finder) at the lower end of the vessel.
Either of the foregoing devices can be used with or without dense
media. Hydrocyclones used without dense media for gravity
separations are referred to as water-only hydrocyclones and those
that are used with dense media are referred to as heavy media
hydrocyclones. The dense media usually consists of an aqueous
suspension of finely ground magnetite or ferrosilicon to control
the specific gravity of the media between the specific gravities of
the two components of the feed material. The finely ground media
material is recovered from both the overflow and the underflow
streams by screening and recycling. This requirement adds to the
cost and complexity of the separation and limits the process with
respect to the size of particles which can be separated.
Additional information regarding hydrocyclone separators and their
operation may be found in the following publications:
The Hydrocyclone, D. Bradley, Pergamon Press, Oxford, 1965;
"Performance of the Hydrocyclone as a Fine-Coal Cleaner", P. Sands,
M. Sokaski, and M. R. Geer, Bureau of Mines Report of
Investigations, 7067, United States Department of the Interior,
January 1968;
"Performance Characteristics of Coal-Washing Equipment;
Dense-Medium Cyclones", A. W. Deurbrouck and J. Hudy, Jr., Bureau
of Mines Report of Investigations, 7673, United States Department
of the Interior, 1972;
"Performance Characteristics of Coal-Washing Equipment;
Hydrocyclones", A. W. Deurbrouck, Bureau of Mines Report of
Investigations, 7891, United States Department of the Interior,
1974; and
"Water-Only Cyclones; Their Functions and Performance", E. J.
O'Brien and K. J. Sharpeta, Coal Age, pgs. 100-114, January
1976.
Surprisingly, it has been discovered that flotation can be
accomplished in a centrifugal field for improved efficiencies in
the recovery of particles particularly with respect to those
particles which are conventionally considered too small to be
recovered by gravity separators and which do not respond well in
conventional froth flotation systems in a gravitational field. Such
an apparatus and method is disclosed and claimed herein.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
The present invention relates to a novel flotation apparatus and
method whereby the flotation is achieved in the centrifugal field
of a hydrocyclone device. The apparatus is configurated as any one
of a variety of suitable, conventional cyclonic separators which
has been suitably modified to accomodate the novel method of this
invention. Air for the flotation separation technique may be
supplied either through a porous wall in the cyclonic device or by
means of air dispersed into a medium introduced into the cyclonic
device.
It is, therefore, a primary object of this invention to provide
improvements in gravity and flotation separation techniques.
Another object of this invention is to provide an improved
hydrocyclone useful as a flotation device.
Another object of this invention is to provide improvements in
flotation techniques.
Another object of this invention is to provide an improved
hydrocyclone having a porous wall surrounding a portion of the body
of the hydrocyclone, the porous wall forming a part of the wall for
an air plenum and serving to introduce air into the
hydrocyclone.
Another object of this invention is to provide an improved
apparatus for introducing finely dispersed air bubbles within a
liquid media for a cyclonic separator and thereby provide the
necessary froth phase for flotation in a centrifugal field.
These and other objects and features of the present invention will
become more fully apparent from the following description and
appendent claims taken in conjunction with the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a chart comparing the percentage of recovery from
specified size intervals with the average particle size of these
intervals for various minerals using standard flotation
techniques;
FIG. 2 is a perspective view of a first preferred embodiment of the
novel apparatus of this invention for obtaining flotation in a
centrifugal field with portions broken away to reveal internal
construction and operation;
FIG. 3 is an enlarged, schematic representation of a fragment of
FIG. 1 to illustrate the novel process of this invention of
flotation in a centrifugal field;
FIG. 4 is a second preferred embodiment of the novel apparatus of
this invention for obtaining flotation in a centrifugal field with
portions broken away to reveal internal construction and operation;
and
FIG. 5 is a third preferred embodiment of the novel apparatus of
this invention for obtaining flotation in a centrifugal field with
portions broken away to reveal internal construction and
operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is best understood by reference to the drawing
wherein like parts are designated with like numerals
throughout.
GENERAL DISCUSSION
From the foregoing prior art publications, and as a result of the
various observations which are significant in relation to the
flotation of fine particles (less than approximately 15
micrometers), the following equation has been reported for fine
particles to explain their flotation response. Advantageously, the
equation offers clues to methods of improving the rate of flotation
of fines. The rate constant, k, is expressed as: ##EQU1## Where
(.beta.) is the proportion of particles retained in the froth after
fruitful collision; (a), is the radius of the bubble, radius of
curvature; (r), is the particle radius; (u), is the relative
particle bubble velocity; (N), is the number of bubbles per unit
volume of pulp; (.lambda.), is the induction time. Inherent in
(.lambda.) are the numerous chemical factors endowing the mineral
surface with appropriate hydrophobic character. All the other terms
relate to the physical environment in a flotation cell, especially
concerning the gas phase; (a), bubble radius or bubble size; (N),
bubble concentration; and (u), relative bubble/particle velocity.
The increase in flotation rate arising from an increase in aeration
rate (N), is well-known.
On first inspection, it would appear that the form of the equation
would seem to predict that the rate constant, k, would increase as
bubble size increases. However, researchers have pointed out that
these predictions tend to contradict practical observations. There
is a common factor that has not been stressed in any of the
foregoing arguments; that is the simultaneous change in both bubble
number, (N), and the average bubble velocity, (u), which will occur
in a real flotation system if any step is taken to adjust average
bubble size. The foregoing Equation 1 indicates how all these
factors are simultaneously involved and a "bubble factor", B, can
be isolated from the rate constant equation as follows:
##EQU2##
Table I presents bubble size, velocity, number, etc., for a
specified flotation system (i.e., 10.5 percent air by volume in the
pulp; 200 bubbles of one millimeter diameter per cubic centimeter
of pulp). Attention is particularly directed to the large increase
in the "bubble factor" and thus, flotation rate constant, as bubble
size decreases. This increase is seen to rise mainly from the large
increase in bubble numbers which completely masks the opposing size
and velocity effects.
TABLE I ______________________________________ Change in `Bubble
Factor` with Bubble Size at Constant Bubble Concentration (.lambda.
= 0.0094 sec) (cm)Diam. (2a)Bubble (cm/sec)Velocity (u)Bubble N
##STR1## B ______________________________________ 0.15 15.5 60
0.195 13.4 0.10 10.2 200 0.202 20.6 0.06 5.5 926 0.261 39.8 0.05
4.4 1600 0.285 50.1 0.04 3.5 3125 0.288 63.0 0.03 2.4 7407 0.343
91.5 0.02 1.3 25000 0.475 154.5 0.01 0.45 200000 0.685 308.3
______________________________________
While theory confirms generally held opinions among metallurgists
that any measure which can be adopted to reduce bubble size will
aid flotation, it has been observed that recovery is very poor in a
flotation column using very fine bubbles. In general, designers of
industrial flotation cells do not appear to have produced a
satisfactory solution to the problem of producing fine bubbles
economically and then using them efficiently.
However, the radial flow of fine gas bubbles in a centrifugal field
of about 80 G results in bubble velocities on the order of 1600
cm/sec. Such conditions are especially well-suited for the
flotation of fine particles and should extend the fine size limit
for flotation in many systems. In addition, the use of an
air-sparged hydrocyclone for coal cleaning is believed to be an
excellent application and experimental results demonstrate its
effectiveness in ash rejection compared to traditional flotation
separation in a gravitational field. Experimental results for other
mineral systems also indicate similar success can be realized even
for systems in which the gravity differential would not generally
be favorable for the separation.
The Embodiment of FIG. 2
Referring now more particularly to FIG. 2, a first preferred
embodiment of the novel apparatus of this invention for achieving
flotation in a centrifugal field is shown generally at 10 as an
air-sparged hydrocyclone. The body of hydrocyclone 10 is
configurated generally as a conventional hydrocyclone having an
upper, cylindrical section 12 and terminating at its lower end in a
downwardly directed cone 18 with an underflow apex 20 for underflow
44. A vortex finder 28 is inserted into cylindrical section 2 and
provides an outlet for an overflow product 32 through an outlet 30.
A feed inlet 24 introduces a slurry feed 38 tangentially into
cylindrical section 12 to thereby create the cyclonic action
therein. A section 22 changes the inlet 23 from a circular
cross-section to the rectangular cross-section for inlet 24.
A porous wall 42 is formed as a wall for a portion of hydrocyclone
10. Porous wall 42 is surrounded exteriorly by an air plenum 40
formed by a cylindrical wall 17 extending between an upper flange
15 and a lower flange 16. An air inlet 34 admits air 36 under
pressure into air plenum 40.
With particular reference also to FIG. 3, air 36 in air plenum 40
is shown schematically as arrows 36a-36c penetrating porous wall 42
and becoming a plurality of discrete air bubbles 48. The slurry
feed 38 includes a plurality of hydrophobic particles 46 and
hydrophilic particles 47 traveling in a counterclockwise cyclonic
action as indicated schematically by arrow 39. Air bubbles 48
attach themselves under known, conventional flotation techniques
and are carried inwardly toward the center vortex of hydrocyclone
10 where they are carried upwardly through the overflow outlet 30
as overflow 32. Importantly, it should be clearly understood that
hydrophobic particles 46 are illustrated schematically herein for
ease of illustration and presentation. With particular reference to
Equation 1 further in combination with Table I, it will be observed
that both the bubble numbers (N), and the average bubble velocity
(u) in a centrifugal field of approximately 80 G should be
sufficient to provide a surprisingly improved flotation of
particles 46 thereby substantially extending the curves of FIG. 1
to the left so that recovery of a significantly smaller particle
size will be achieved.
The foregoing principles with respect to FIG. 3, although presented
herein with respect to the first preferred embodiment illustrated
in FIG. 2, are clearly applicable throughout this discussion and
also particularly with respect to the second and third preferred
embodiments of this invention shown in FIGS. 4 and 5,
respectively.
The Embodiment of FIG. 4
Referring now more particularly to FIG. 4, a second preferred
embodiment of the novel apparatus of this invention for achieving
flotation in a centrifugal field is shown generally at 50 and
includes a cylindrical vessel 52 having a coaxial inlet 54 for a
feed 55 at an upper end and a coaxial outlet 56 for a product
discharge 57 at the lower end. A portion of the external wall of
vessel 52 is formed as a porous wall 60 which is surrounded by an
air plenum 58 formed by a cylindrical wall 59 cooperating between
upper and lower flanges 64 and 65, respectively. An air inlet 62
provides access for pressurized air 63 into air plenum 58.
Cyclonic action in vessel 52 is created by a tangentially arrayed
wash water inlet 66 for wash water 67 under pressure. Wash water 67
entering vessel 52 rotates in a counterclockwise direction as
indicated schematically by broken arrow 67a and travels upwardly
through the interior of vessel 52 to a second tangential outlet,
sink discharge outlet 68 where it becomes sink discharge 69. The
cyclonic action of wash water 67 as shown by broken arrow 67a
creates a corresponding vortex for feed 55 thereby resulting in the
more dense particles in feed 55 being carried over by wash water 67
to sink discharge 69. Lighter particles continue with feed 55 in an
inner vortex, indicated schematically at broken line 55a, are
discharged through outlet 56 as product discharge 57. The general
transition line between the two vortices is shown schematically by
broken line 51.
Referring also to the discussion hereinbefore with respect to the
schematically illustrated process of FIG. 3, air 63 passing into
air plenum 58 is directed through porous wall 60 thereby forming a
plurality of discrete bubbles (schematically similar to bubbles 48,
FIG. 3) to achieve the novel flotation process in a centrifugal
field of this invention.
The Embodiment of FIG. 5
Referring now more particularly to FIG. 5, a third preferred
embodiment of the novel apparatus of this invention for achieving
flotation in a centrifugal field is shown as cyclonic flotation
separator 80. Cyclonic flotation separator 80 is configurated as a
cylindrical vessel 82 having a coaxial, feed inlet 84 at an upper
end for a feed stream 85 and a corresponding, coaxial outlet 86 at
a lower end for product discharge 87. Cyclonic action in vessel 82
is created by wash water 95 being tangentially introduced into
vessel 82 by a tangential inlet 92. The flow pattern thus created
is schematically illustrated at broken lines 95a as a cyclonic
vortex. The cyclonic vortex in vessel 82 directs wash water 95
upwardly through vessel 82 to discharge outlet 88 as sink discharge
89. The corresponding cyclonic action of feed 85 as generated by
wash water 95 is shown at vortex 85a (shown in broken lines) with
the region between the vortices being indicated generally with
broken lines as column 81. Air, indicated schematically at arrow
97, is introduced through an inlet 96 into a mixer 90 where it is
intimately blended as a fine dispersion of bubbles (see bubbles 48,
FIG. 3) in wash water 95. Mixer 90 can be of any suitable
configuration and may include, for example, an externally-powered
mixing apparatus for achieving the fine dispersion of bubbles 48
(FIG. 3) in the process. Alternatively, gas bubbles 48 (FIG. 3) may
be generated electrolytically or by any other suitable process.
The invention may be embodied in other specific forms without
departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative and not restrictive and the scope of the invention is,
therefore, indicated by the appended claims rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
* * * * *